Carbon-carbon composites are members of a family of materials which consist of carbon fibres in a carbon matrix. These materials have desirable properties, particularly at high temperatures. For example the strength is not only retained at temperatures up to 2000.degree. C. but actually increases compared to its value at room temperature. However, in oxidizing atmospheres at temperatures above 500.degree. C. the materials oxidize, thereby rapidly losing mass and strength.
Previous attempts have been made to prevent oxidation by various means such as additives and coatings, with varying degrees of success. A known coating system employs an impervious refractory material such as silicon carbide or silicon nitride. One problem with such a material is that its coefficient of thermal expansion is much higher than the coefficient of thermal expansion of carbon-carbon composite in the axial direction of the carbon fibres. Consequently coatings of the refractories tend to crack or spall as the result of differential expansion or contraction of the composite and its coating when the composite is subjected to temperature changes. This problem exists in composites of many kinds including those in which the fibres are randomly arranged, but is of particular concern where the fibres are arranged substantially aligned to lie in a common plane, and more so when they are aligned to lie substantially all in a common direction.